In order to meet the emission level requirements, for industrial low emission gas turbine engines, staged combustion is required in order to minimise the quantity of the oxides of nitrogen (NOx) produced. The current emission level requirement, in some countries, is for less than 25 volumetric parts per million of NOx for an industrial gas turbine exhaust. One fundamental way to reduce emissions of nitrogen oxides is to reduce the combustion reaction temperature, and this requires premixing of the fuel and the combustion air before combustion occurs. The oxides of nitrogen (NOx) are commonly reduced by a method which uses two stages of fuel injection. Our UK patent no. GB1489339 discloses two stages of fuel injection. Our International patent application no. WO92/07221 discloses two and three stages of fuel injection. In staged combustion, all the stages of combustion seek to provide lean combustion and hence the low combustion temperatures required to minimise NOx. The term lean combustion means combustion of fuel in air where the fuel to air ratio is low, i.e. less than the stoichiometric ratio. In order to achieve the required low emissions of NOx and CO it is essential to mix the fuel and air uniformly.
The industrial gas turbine engine disclosed in our International patent application no. WO92/07221 uses a plurality of tubular combustion chambers, whose axes are arranged in generally radial directions. The inlets of the tubular combustion chambers are at their radially outer ends and transition ducts connect the outlets of the tubular combustion chambers with a row of nozzle guide vanes to discharge the hot gases axially into the turbine sections of the gas turbine engine. Each of the tubular combustion chambers has two coaxial radial flow swirlers which supply a mixture of fuel and air into a primary combustion zone. An annular secondary fuel and air mixing duct surrounds the primary combustion zone and supplies a mixture of fuel and air into a secondary combustion zone.
One problem associated with gas turbine engines is caused by pressure fluctuations in the air, or gas, flow through the gas turbine engine. Pressure fluctuations in the air, or gas, flow through the gas turbine engine may lead to severe damage, or failure, of components if the frequency of the pressure fluctuations coincides with the natural frequency of a vibration mode of one or more of the components. These pressure fluctuations may be amplified by the combustion process and under adverse conditions a resonant frequency may achieve sufficient amplitude to cause severe damage to the combustion chamber and the gas turbine engine.
It has been found that gas turbine engines which have lean combustion are particularly susceptible to this problem. Furthermore it has been found that as gas turbine engines which have lean combustion reduce emissions to lower levels by achieving more uniform mixing of the fuel and air, the amplitude of the resonant frequency becomes greater. It is believed that the amplification of the pressure fluctuations in the combustion chamber occurs because there is instability in the combustion process, there is a resonant cavity and the heat released by the burning of the fuel occurs at a position in the combustion chamber which corresponds to an antinode, or pressure peak, in the pressure fluctuations.
It is also known to provide gas turbine engine combustion chambers which have a plurality of catalytic reaction zones arranged in series to minimise nitrous oxide (NOx) emissions. One known arrangement is described in our European patent application EP0805309A, published Nov. 5, 1997 . In this arrangement a pilot injector is provided to burn some of the fuel to preheat a first catalytic reaction zone to its operating temperature. A main injector is positioned upstream of the first catalytic reaction zone to supply fuel to the first catalytic reaction zone. The second and subsequent catalytic reaction zones receive unburned fuel from the first catalytic reaction zone.
A problem with this arrangement is that it does not fit into the space available, and it requires staged fuelling between the catalytic reaction zones.
It is also known to provide gas turbine engine combustion chambers which have staged combustion using combustion of lean fuel and air mixtures in a catalytic reaction zone downstream of the last staged combustion zone and a homogeneous combustion zone downstream of the catalytic reaction zone to further reduce emissions of NOx. One known arrangement is described in our European patent application no. EP0810405A, published Dec. 3, 1997.
It is also known to provide catalytic partial oxidation in which a hydrocarbon fuel is mixed with air so that rich combustion occurs in contact with a catalyst to form a product gas which comprises a mixture of hydrogen, carbon monoxide, water, carbon dioxide and unreacted hydrocarbon fuel. The hydrocarbon fuel is burned with insufficient amounts of oxygen, for complete oxidation, such that it is only partially oxidised. The term rich combustion means combustion of fuel in air where the fuel to air ratio is high, i.e. greater than the stoichiometric ratio for complete oxidation. International patent application no. WO92/20963, published Nov. 26, 1992 describes a combustion system for a gas turbine where all the fuel is supplied to a catalytic partial oxidation reaction zone, the product gas of the catalytic partial oxidation reaction zone are mixed with air and supplied to a primary combustion zone and finally the products of the primary combustion zone are mixed with air and supplied to a secondary combustion zone. This arrangement reduces NOx emissions.